How to spot a bully during an interview?

I watch out for people who repeatedly ask the same question without adding any clue or context even when I state I do not understand the question or do not have experience with the thing they are asking about. They won't let you move forward and discuss the things you do have experience with.

Watch out for interviewers who spend a lot of time talking about themselves and their accomplishments.

Watch out for interviewers who make negative comments about your current or past employer.

For me, this is quite difficult. One thing you could look for is their response to any hint that they were incorrect or ignorant on something. Good people will become curious and drill down, hoping to discover something new. Bad people will become defensive or stonewall.

More importantly, though, you should have a plan for gracefully leaving the company within the first month or six, should you discover that your boss is a bully. Don't put yourself in the position of being "stuck" financially (or in other ways).

If they ask loaded questions / trick questions that are irrelevant to the interview and the job description.

Some friend of mine was interviewing for a junior position at some startup in London and he noticed one of the interviewers secretly recording him while asking a strange whiteboard question all from memory which they don't use in production at all. The interviewers ridiculed and belittled him straight through the interview with the recording and he told me he found the place very unwelcoming.

The thing is, it is usually the self proclaimed 'senior engineers' who have less than 4 years of experience at a single company who have also been bullied that are most likely to be this arrogant and cannot ask the right questions, thus cannot interview juniors properly; only to feel better about themselves afterwards.

After this he told me that the company in question raised two series rounds in less than 5 months; a statment that they're haemoraging cash. I'd say he dodged an unprofitable bad egg anyway.

Irrelevant and sometimes technically incorrect questions asked repeatedly or asking a question again that you failed to answer to their satisfaction. I only had it happen to me once in what was meant to be the non-technical interview part of a long interview process by a clueless CTO. In the previous technical section of the interview he had asked a very strange question regarding a solution I had presented and was corrected by a member of his own dev team, I think from that point on he took a dislike to me.

I've had many interviews, but only one I would label a bully. His tone of voice and body language came off as angry, hostile, and annoyed. He also kept making phone calls during the interview. At the time, I was still a fresh undergrad without any experience so I just tolerated his attitude, but in hindsight it was obvious that he was being disrespectful and I should have just excused myself.